Preparation of polymeric membranes for fluid and gas separation applications are well documented in the art. In order for a polymer to qualify as a suitable membrane forming and fluid separation material, it must meet several performance criteria that will depend on the end use of the membrane. Among the factors that will influence the choice of a polymer are its mechanical strength, chemical resistance, thermal stability, and most importantly its separation and permeation characteristics. In addition to the aforementioned considerations, it is frequently preferred that a prospective membrane polymer be commercially available at a moderate cost.
Polysulfones are materials that frequently meet these requirements. Israel Cabasso documents techniques for making polysulfone membranes in the form of hollow fibers in. "Hollow Fiber Membranes", Kirk-Othmer Encyclopedia of Chemical Technology, 12, Third Edition, 492-519 (1980), and "Membranes", Encyclopedia of Polymer Science and Engineering, 9, Second Edition, 509-579 (1987). Polysulfone membranes are described extensively in the literature and are used in many commercial fluid and gas separation applications. However, there are applications in which the use of a polysulfone membrane would be unsuitable. These applications often require operating conditions that exceed the chemical, mechanical, or thermal properties of polysulfone membranes. The fluid separation characteristics of polysulfone membranes, in particular the gas separation and permeation characteristics of polysulfones, are frequently limited. These higher performance requirements are frequently met by polyimide membranes.
Polyimides frequently exhibit exceptionally good thermal resistance as well as chemical and mechanical properties that exceed those of polysulfone. Advanced performance polyimides are expensive, and consequently their use as membrane materials is often limited. One polyimide based material that can be used economically is Ultem.RTM. 1000, a polyetherimide manufactured by General Electric. This polyetherimide polymer is of interest as a membrane forming material because of its superior strength and chemical resistance.
The manufacture of integral asymmetric polyetherimide membranes is taught by Peinemann in U.S. Pat. No. 4,673,418. Kneifel et al. in U.S. Pat. Nos. 4,818,452 and 4,933,085 describe processes for making polyetherimide hollow fiber membranes capable of separating liquid or gaseous mixtures. Kneifel and Peinemann further report the utility of polyetherimide hollow fiber gas separation membranes in an article in the Journal of Membrane Science, 65 (1992), 295-307.
There are numerous references in the literature to membranes manufactured from polyimides. Makino et al. teach preparation of several specially polyimides and membranes produced therefrom in U.S. Pat. Nos. 4,440,643; 4,460,526; 4,512,893; and 4,528,004. Chung et al. describe preparation of asymmetric hollow fibers for gas separation from fluorine containing polyimides in the Journal of Membrane Science, 75 (1992), 181-195. This work is an example of a gas separation membrane fabricated from an expensive, custom synthesized polymer.
Another material with advantageous properties is a polyimide that incorporates phenylindane moieties in the polymer chains. This polymer is sold by Ciba Geigy under the trade name of Matrimid.RTM. 5218. This material has a good combination of gas permeability coefficients and separation factors for many gas pairs. The fabrication of asymmetric membranes from this polyimide has been reported by Wang et al. in U.S. Pat. No. 5,067,970. Ekiner et al. disclose the use of phenylindane containing polyimides to prepare gas separation membranes in U.S. Pat. No. 5,015,270. While membranes described in the aforementioned patents display good gas separation characteristics, commercial use of these membranes can be limited because of the high cost of this specialty polymer. These specialty polymers are frequently chosen as membrane materials because of their enhanced ability to separate gases. The improved performance can manifest itself in the form of higher permeation rate for a particular gas or an increased separation factor for one or more pair of gases. The polyetherimide polymer Ultem.RTM. 1000 has good intrinsic separation factors for many gas pairs including O.sub.2 /N.sub.2 and CO.sub.2 /CH.sub.4 ; however, the fast gas permeation rates for this polymer are low. Thus, there continues to be a need to improve gas permeation characteristics of polyimide based membrane systems in an economical manner.
One approach that has been disclosed in the art to fill this need is the coextrusion process. To minimize the amount of specialty polymer used in membrane preparation, hollow fibers have been prepared by this process with an inner core and an outer sheath that consists of two different materials. The core, which typically constitutes the majority of the fiber volume, is composed of a polymer that merely acts as a porous support for the sheath polymer. Thus, the core material may be selected from any number of common polymers with adequate mechanical and thermal characteristics. The separation layer of the membrane is formed by the sheath polymer with optimal separation/permeation characteristics that preferably make up only a fraction of the fiber volume. Tsujii et al. in Japanese Patent Application Sho61-32261 employ a coextrusion process to produce gas separation membranes from a variety of polymers including cellulose esters. Kusuki et al. in Japanese Patent Application Sho62-253785 report preparation of polyimide gas separation membranes by coextrusion. The use of phenylindane containing polyimides as the sheath layer in a coextruded hollow fiber is reported by Ekinr et al. in U.S. Pat. No. 5,085,676. Example 40 of this patent further discloses the use of Ultem.RTM. 1000/Matrimid.RTM. 5218 blend in 75:25 weight ratio as the core layer in coextruded hollow fibers. It is stated that the blend is used to promote adhesion between the core and sheath layers but does not act as a separation material. The polymer blend core layer does not contain an integral discriminating layer and serves only as a support for the sheath layer. The use of Ultem/Matrimid blend in 90:10 weight ratio as a core layer in coextruded hollow fibers is further disclosed in U.S. Pat. No. 5,248,319. The use of polyetherimide/phenylindane containing polyimide blend as an integral asymmetric gas separation membrane is disclosed by Ekiner and Simmons in U.S. Pat. No. 5,248,319. Ultem/Matrimid polymer blends of 75:25 ratio are spun into integral asymmetric hollow fibers (comparative example 1, column 12). The Ultem/Matrimid polymer blend ratio utilized by Ekiner and Simmons is relatively high to be attractive economically due to the high cost of Matrimid polymer, and furthermore, the utility of this blend to prepare membranes with integral discriminating layers of superior separation/permeation performance is not recognized. The permeation properties of hollow fiber membranes prepared from Ultem/Matrimid blend were referred to as not being attractive (column 13, lines 1 through 5).
There are numerous additional examples in the art of fluid separation membranes advantageously prepared from blends of polymers. Kraus et al. in U.S. Pat. No. 5,076,935 teach the use of polyethersulfone/phenoxy resin blends to make porous isotropic membranes. Nunes et al. in the Journal of Membrane Science, 73 (1992), 25-35, describe the preparation of asymmetric membranes useful for ultrafiltration from blends of polyvinylidene fluoride and polymethyl methacrylate. The practice of blending polymers has also been used effectively in the formation of gas separation membranes. Burgoyne, Jr. et al. in U.S. Pat. No. 5,061,298 disclose the use of blends of polyimide polymers as part of a process to prepare air separation membranes as shown. Yamada et al. in U.S. Pat. No. 4,832,713 disclose fabrication of gas separation membranes from blends of polyetherimide mixed with materials such as polycarbonates or polysulfones. However, the prior art does not disclose preparation of integral asymmetric fluid separation membranes with superior combination of separation/permeation characteristics from blends of polyetherimide and phenylindane containing polyimides wherein the amount of phenylindane containing polyimide polymer in the blend is low or the use of such blends for preparation of porous substrates useful in the manufacture of composite membranes by solution coating processes.
The approaches disclosed in the prior art to produce polyimide membranes in an expedient and economical manner have been somewhat deficient. Coextrusion takes significant steps in correcting prior art deficiencies, but is a cumbersome process requiring substantial investment in specialized hardware such as spinnerettes. Furthermore, coextrusion processes can consume significant amounts of the separation layer polymer since it must: completely encircle the core polymer at a thickness capable of promoting the integrity of the separating surface. Thus, there still remains a need for preparation of improved fluid separation membranes from polyimide polymers.